In integrated circuits, contact plugs are used for connecting to the source and drain regions, which may be formed through epitaxy. The source/drain contact plugs are typically connected to source/drain silicide regions. The formation of the source/drain silicide regions includes forming contact openings by etching dielectric layers covering the source/drain regions, wherein the etched dielectric layers may include a silicon nitride layer and an oxide layer over the silicon nitride layer. The source/drain regions are thus exposed to the contact openings. An additional silicon nitride layer is formed conformally to cover the sidewalls and the bottoms of the contact openings. A second etching step is then performed to remove the bottom portions of the silicon nitride layer to reveal the epitaxy source/drain regions. A metal layer is then formed to extend into the contact openings, and an anneal is performed to react the metal layer with the source/drain regions, resulting in source/drain silicide regions to be formed. The remaining portions of the contact openings are then filled with a metal(s) to form the source/drain contact plugs.
In conventional processes for forming the contact openings, the etching of various dielectric layers to expose the source/drain regions results in the loss of highly-doped epitaxy source/drain regions. Specifically, the topology of the epitaxy source/drain regions may be lost, and the top surfaces of the epitaxy source/drain regions become flatter, resulting in the area of the source/drain silicide regions to be smaller. The loss of highly doped epitaxy source/drain regions, together with reduced contact area, increases the contact resistance of the source/drain regions. This impacts circuit speed and performance.